1. Technical Field
The present invention relates to a liquid crystal display apparatus for displaying an image, and more specifically to a liquid crystal display apparatus of a transverse electric field mode capable of enlarging a viewing angle.
2. Description of the Related Art
In recent years, as a display mode for a liquid crystal display apparatus, a transverse electric field mode represented by Fringe-Field Switching (FFS) or In-Plane Switching (IPS) with preferable viewing angle characteristics has widely been employed. Moreover, for a television receiver, in addition to the screen resolution of Full High Definition (FHD, 1920×1080 pixels) which is a current mainstream, the screen resolution of 4K2K (3840×2160 pixels) is becoming widespread. Furthermore, in the near future, the screen resolution of 8K4K (7680×4320 pixels) is expected to rise in the field. It is predicted that the screen size is enlarged as the resolution is increased, and therefore the demand for enhancement in the viewing angle characteristics may also be further increased.
In a liquid crystal display apparatus of the IPS mode, in the front view, two types of comb-like electrodes (pixel electrode and common electrode) are arranged at intervals. An electric field is applied between the pixel electrode and the common electrode so as to drive liquid crystal molecules. The alignment of liquid crystal molecules will not show a large change even if the viewing angle is changed, which allows an observer to change the viewing direction without a large change in the display, thereby presenting preferable viewing angle characteristics. In a liquid crystal display apparatus of the FFS mode, on the other hand, a planar common electrode and a smaller pixel electrode are arranged with an insulating layer interposed in between. An electric field called a fringe electric field applied between the pixel electrode and the common electrode is used to drive liquid crystal molecules. In the FFS mode, since the fringe electric field includes more electric fields having components in a direction orthogonal to a substrate compared to the IPS mode, liquid crystal molecules are more likely deformed in unintended vertical directions. This deteriorates the viewing angle characteristics and thereby degrading the viewing angle characteristics compared to that in the IPS mode. This is a difference between the IPS and FFS, both of which are the types of the transverse electric field mode.
Japanese Patent Application Laid-Open Publication No. 2009-186869 proposes a liquid crystal display apparatus of the IPS mode also provided with the characteristics of the FFS mode. FIG. 1 is a schematic front view of the liquid crystal display apparatus described in Japanese Patent Application Laid-Open Publication No. 2009-186869 from which a color filter substrate is excluded, illustrating a part corresponding to one subpixel. A similar drawing is illustrated in FIG. 1 of Japanese Patent Application Laid-Open Publication No. 2009-186869. FIG. 2 is a cross-section view along the line VI-VI in FIG. 1. A similar drawing is illustrated in FIG. 3 of Japanese Patent Application Laid-Open Publication No. 2009-186869. A liquid crystal layer 130 is disposed between an array substrate AR and a color filter substrate CF. The array substrate AR, the color filter substrate CF and the liquid crystal layer 130 are arranged between a first polarization plate 131 and a second polarization plate 132. The array substrate AR includes the first transparent substrate 111 on which a gate insulating film 114 is formed, and a data line (signal line) 113 as well as a passivation film 115 are further formed on the gate insulating film 114. A planarized film 116 is formed on the passivation film 115. A third electrode 118 is disposed solidly on the planarized film 116, while an insulating film 119 is formed on the third electrode 118. On the insulating film 119, a first electrode 121 and a second electrode 122 both having comb-like shapes are arranged with a clearance 120 in between. The first electrode 121 and the second electrode 122 are covered with a first alignment film 124. The color filter substrate CF includes a second transparent substrate 125, a light shielding film 126, a color filter film 127, an overcoat film 128 and a second alignment film 129.
The first electrode 121 is electrically connected to a drain electrode 133 of a TFT 134 through the first contact hole 117 formed in the insulating film 119, planarized film 116 and passivation film 115. The second electrode 122 is electrically connected to a third electrode 118 through the second contact hole 123 formed in the insulating film 119. An electric field E1 is applied between the first electrode 121 and the second electrode 122, and an electric field E2 is applied between the first electrode 121 and the third electrode 118. As the first electrode 121 and the second electrode 122 are on the same plane, the movement of liquid crystal molecules by the electric field E1 is similar to that in the liquid crystal display apparatus of the IPS mode. Moreover, as the first electrode 121 and the third electrode 118 are overlapped with each other in the front view via the insulating film 119, the movement of liquid crystal molecules by the electric field E2 is similar to that in the liquid crystal display apparatus of the FFS mode. Japanese Patent Application Laid-Open Publication No. 2009-186869 describes that a liquid crystal display apparatus with a bright display having advantages of both IPS and FFS may be obtained by being provided with functions of both modes, in which a high aperture ratio as well as high luminance may be obtained while image persistence and flickering may be suppressed.
Japanese Patent Application Laid-Open Publication No. 2008-39806 proposes a technique of improving viewing angle characteristics at high gradation levels in the FFS mode. FIG. 3 is a front view of the liquid crystal display apparatus described in Japanese Patent Application Laid-Open Publication No. 2008-39806, illustrating a part corresponding to one subpixel. FIG. 4 is a cross-section view along the line VII-VII in FIG. 3. A first common electrode 321, a second common electrode 322 and a gate insulating film 304 are formed on the substrate 301, while a capacitance electrode 307 and an interlayer insulating film 308 are formed on the gate insulating film 304. A planar pixel electrode 310 is formed on the interlayer insulating film 308. At the pixel electrode 310, a slit 3101 and a comb electrode 3102 are formed. The pixel electrode 310 is covered with an alignment film 311. The pixel electrode 310, the first common electrode 321 and the second common electrode 322 are formed of transparent electrodes. On the color filter substrate 313, a color filter 314 and an alignment film 311 are laminated. A liquid crystal layer 3121 is arranged between two alignment films 311. On a scanning line (gate line) 303, an amorphous Si film 305 is formed via the gate insulating film 304, and a drain electrode 351 as well as a source electrode 352 connected to the amorphous Si film 305 are also formed. The pixel electrode 310 is connected to the drain electrode 351 via a through hole 309. The source electrode 352 is integrally formed with the data line (source line) 306. The first common electrode 321 is connected to a common wiring 323.
A common voltage which is a constant voltage is applied from the common wiring 323 to the first common electrode 321. While the second common electrode 322 is a float electrode, it is capacitively coupled with the first common electrode 321 via the capacitance electrode 307. As illustrated in FIG. 4, the capacitance electrode 307 is formed on the gate insulating film 304 while being planarly overlapped with the first common electrode 321 and the second common electrode 322. The potential of the capacitance electrode 307 is a float potential. A capacitance Cb1 is formed between the capacitance electrode 307 and the first common electrode 321, while a capacitance Cb2 is formed between the capacitance electrode 307 and the second common electrode 322.
As illustrated in FIG. 4, the electrical flux lines from the upper surface of the comb electrode 3102 of the pixel electrode 310 permeate the liquid crystal layer 3121 and extend toward the first common electrode 321 and the second common electrode 322. The potential of the second common electrode 322 for moving liquid crystal corresponds to the potential obtained by capacitively dividing the potential of the pixel electrode 310 and a common potential by the capacitance between the second common electrode 322 and the pixel electrode 310 through the liquid crystal and by the capacitance between the second common electrode 322 and the first common electrode 321 through the capacitance electrode 307.
Accordingly, two electric field for driving liquid crystal are present in the same pixel. That is, as illustrated in FIG. 4, an electric field Ec acting on the liquid crystal above the second common electrode 322 near the center of the pixel and a larger electric field Ep acting on the liquid crystal above the first common electrode 321 are located at either side of the second common electrode 322. Japanese Patent Application Laid-Open Publication No. 2008-39806 describes that the viewing angle characteristics are improved by the two different electric fields Ec and Ep (Ec<Ep), mainly due to the electric field Ec being smaller than the electric field Ep.